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Subject: Re: dorsal filaments?
From: Pete Liptrot <[log in to unmask]>
Reply-To:Scientific forum on fish and fisheries <[log in to unmask]>
Date:Tue, 8 Oct 2002 23:18:20 +0100

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> Does anyone have any good hypotheses about the function of the elongated
> dorsal filaments that are found in some fish species?

I've heard various suggestions for this depending on species, some of which
have multi-purpose explanations, and only the first one has any likely
relevance to Tarpon, if any.

In Merodontodus (Pimelodidae), Leptodoras (Doradidae), Pterosturisoma
(Loricariidae), Lamontichthys (Loricariidae) and others, including I imagine
Tarpon, the suggestion has been of 'trailing edge vortices' being used in
hydrodynamism. Turbulence is apparently directed away from the body of the
fish giving more stability during locomotion.
This came some years ago from someone who worked for British Aerospace and
also happened to like Catfish. All these fish either swim at high speeds or
live in high velocity currents.

Visual sign of sexual fitness is another suggested purpose for these.
Satanoperca, Acariichthys, Biotodoma, Biotoecus (all Cichlidae), Panaque
(Loricariidae) frequently display extended filaments from the dorsal and/or
caudal in dominant individuals which become particularly elongate during
reproductive episodes. These filaments may be specifically targeted in
competing individuals as a means of impairing sexual success.

Sacrificial tissue has been another proposed function. In environments where
fin-eating parasite fish species are present it could be useful to have
easily regrown filaments that increase the apparent size of the fins most
frequently attacked. Examples could be the already mentioned Neotropical
Geophagine Cichlid spp. (fin-eating parasites of these are various
Serrasalmines), and also Synodontis decorus (Mochokidae) from the Congo
region. This species develops long tail and dorsal filaments in captivity
and shares a tail pattern with a fin-eating Ichthyoborus found in the same
waters (this could represent aggressive mimicry by the Ichthyoborus sp.,
another unproven theory!).

Yet another I've heard has been in the case of fish species that are
predated upon by largely tactile-hunting predators. The presence of long
filaments could be used when a predator is near to increase the apparent
size of the prey and so dissuade attack

The final purpose I've heard suggested in the case of Sturisoma spp.
(Loricariidae) is a tactile function. In captivity, Sturisoma spp. can be
seen to bend their tail around in front of their head and use the filaments
apparently to 'feel' around. Males can also be seen to apparently assess the
size of one another using these tail extensions in a similar fashion, the
suggestion being that they judge whether it is worth pursuing further
conflict over territory or females.

I'd very much like to hear any thoughts on these ideas.

> Would removing a portion of the dorsal filament of a tarpon (for DNA
> analysis) have an impact on the fish?

Doubt it would to any degree. Soft fin tissue usually regrows very quickly.


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